A conventional uninterruptible power supply system and an uninterruptible power supply system according to the present invention for a single phase circuit share some common elements. In this respect, the conventional power supply system will be described in reference to FIGS. 1 and 2. The primary difference between the two is in the control circuit 20, 20a. 
The conventional power supply system also includes an input relay positioned between input terminals (input end) 11, to which an alternating-current power source 1 is input, and output terminals (output end) 12, to which a load 6 is connected. By placing the input relay 7 in a cutoff (non-conducting) state when an abnormality occurs in the alternating-current power source 1, the alternating-current power source 1 is isolated from the uninterruptible power supply system. A parallel converter is connected in parallel to the input terminals 11 through the input relay 7 and is capable of carrying out a rectifier operation and an inverter operation. Moreover, a series inverter 3 is connected in series between the input relay 7 and one of the output terminals 12 to carry out an inverter operation. Between the parallel converter 2 and series converter 3, a smoothing capacitor 4 and an energy storing unit 5, such as a battery are connected in parallel thereto.
According to the conventional power supply system a control circuit (not illustrated) controls the parallel converter 2 and the series inverter 3. The conventional control circuit, based on an alternating-current power source voltage Vin, supplied from the alternating-current power source 1 and detected by a voltage detector 8, and a terminal voltage Vc of the smoothing capacitor 4 detected by a voltage detector 9, controls the output voltage to the load 6 to become a specified voltage. FIG. 2 illustrates a parallel converter 2 and a series inverter 3 arranged with a full bridge circuit using semiconductor switches (i.e., transistors). The ON-OFF operations of these semiconductor switches are, with respect to the parallel converter 2, carried out based on a gate signal PWM 1 output from the control circuit. Moreover, with respect to the series inverter 3, the operations are carried out based on a gate signal PWM 2 output from the control circuit. On the output side of the parallel converter 2 or the series inverter 3, as shown in FIG. 2, a filter, which is constituted with reactors L and a capacitor C, can be disposed.
When the voltage detector 8 detects that the alternating-current power source voltage Vin of the alternating-current power source 1 is within a permissible range, the input relay 7 is at the ON-state, and the parallel converter 2 operates to charge and discharge the smoothing capacitor 4 based on the terminal voltage Vc of the smoothing capacitor 4 detected by the voltage detector 9 so that the voltage Vc becomes the specified voltage. This converts the alternating-current power of the alternating-current power source 1 to the direct-current power so that the terminal voltage Vc of the smoothing capacitor 4 is maintained at the specified voltage. In addition, the series inverter 3 is operated to output a compensating voltage ΔV to compensate for any excess or deficiency in the alternating-current power source 1 so that a voltage applied to the load 6 becomes the specified voltage. This converts the direct-current power from the smoothing capacitor 4 to the alternating-current power with the applied voltage to the load 6 maintained at the specified voltage.
When the alternating-current power source voltage Vin from the alternating-current power source 1 is outside the permissible range, it is decided that an abnormality has occurred on the alternating-current power source side, the input relay 7 is switched into a cutoff state and the operation of the series inverter 3 is stopped, while the parallel converter 2 is made to perform an inverter operation to convert the direct-current power of the energy storing unit 5 to a specified alternating-current power, which is supplied to the load 6. This makes the applied voltage to the load 6 to be maintained at the specified voltage even when the alternating-current power source side is isolated. See for example, JP-A-2002-199620.
In the conventional uninterruptible power supply system, however, when the input relay 7 is formed with mechanical contacts, an operation delay of several milliseconds to ten and some odd milliseconds occurs in the input relay 7 from the inception of an ON-OFF command to an actual completion of operation. Therefore, from the occurrence of the power failure to the isolation of the alternating-current power source side from the uninterruptible power supply system 100, during which the power failure is detected, while the input relay 7 is controlled to be actually brought to the cutoff state, the load side of the input relay 7 can experience power failure or drop out. In this state, where the power supply to the load is continued with the operation of the series inverter 3 stopped and the parallel converter 2 made to carryout an inverter operation in response to the detection of power failure, when the alternating-current power source voltage Vin is outside the permissible range by occurrence of short circuit on the alternating-power source side, for example, the alternating-power source side in the short circuit state brings the parallel converter 2 connected to the power source in parallel, also into the short circuit state. Therefore, it makes it impossible to maintain the voltage applied to the load 6 at the specified voltage.
Moreover, an operation of the parallel converter 2 for maintaining the specified voltage produces an electric potential difference between the power source voltage after the power failure and the output voltage of the parallel converter 2. This, in some cases, causes current to flow between the alternating-current power source 1 and the parallel converter 2. In particular, at a short circuit failure occurred at the short circuit of the alternating-current power source 1, an unexpected short circuit current can flow in some cases, interfering with the whole uninterruptible power supply system. Furthermore, when the alternating-current power source side is brought into an open circuit state by the power failure, there is also a problem in which, although the alternating-current power source side is essentially in a state without power source, a voltage is supplied thereto from the uninterruptible power supply system.
Accordingly, there remains a need to remove the above-described problems associated with a conventional system so that an uninterruptible power supply system can avoid dropout in an output voltage when the alternating-current power source side is isolated. The present invention addresses this need.